Reilly was born in Dundee to Irish parents. He was a bicycle trader and manufacturer together with his father and his two younger brothers. He became manager/secretary from the initial forming of Dundee Hibernian in 1909. A two-year spell aside, when he remained club secretary, Reilly was in charge for the first thirteen years of Dundee Hibs' existence, leaving just before the name change to Dundee United. It is widely acknowledged that Reilly was instrumental in forming the club, and that without him, there would have been no Dundee United.